netdev - Re: [PATCH net-next] net: move inet_dport/inet_num in sock

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Date:	Wed, 28 Nov 2012 03:27:41 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Ling Ma <ling.ma.program@...il.com>
Subject: Re: [PATCH net-next] net: move inet_dport/inet_num in sock_common

On Tue, 2012-11-27 at 20:12 -0800, Eric Dumazet wrote:

> The point of having the cond jump on sk_hash/hash was that in one
> compare, we catch the yes/no status with 99.999999 % success rate.
> 
> All the following compares are predicted by the cpu and essentially are
> free. Adding the AND or OR will basically have the same cpu cost.
> 
> If we wanted to do a full test of all tuple fields and a single
> conditional jump, we would not have to include hash test at all.
> 
> (If the 4-tuple matches, then sk_hash/hash value _must_ be the same by
> definition)

What I am going to do is to remove the hash compare from the macros so
that we can use likely()/unlikely() to explicitly give hints to the
compiler.

The hash compare can be omitted in the validation done after the
atomic_inc_not_zero() [ done to make sure keys didnt change ]

begin:
        sk_nulls_for_each_rcu(sk, node, &head->chain) {
                if (sk->sk_hash != hash)
                        continue;
                if (likely(INET_MATCH(sk, net, acookie,
                                      saddr, daddr, ports, dif))) {
                        if (unlikely(!atomic_inc_not_zero(&sk->sk_refcnt)))
                                goto begintw;
                        if (unlikely(!INET_MATCH(sk, net, acookie,
                                                 saddr, daddr, ports, dif))) {
                                sock_put(sk);
                                goto begin;
                        }
                        goto out;
                }
        }
 

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